This application pertains to the field of liquid ink printing systems such as ink jet printers, and in particular, to methods and apparatus for improving the quality of a printed product produced by an ink jet printer by reducing printed edge roughness.
Thermal ink jet print heads, commonly called "pens", print by expelling drops of a liquid printing solution such as ink onto a planar substrate such as paper or transparency material. The pen, mounted on a carriage, is transported horizontally across the paper surface, spaced just slightly apart from the surface. Vertical positioning is achieved by moving the paper up or down on the platen. Through a combination of moving the pen and paper, ink drops are deposited onto the paper in a predetermined pattern so as to form a desired image which may include characters and/or graphics.
In such a system, the ink is expelled from the pen by explosively boiling it, while the pen travels across the surface of the paper. These drops or jets of ink break up into a primary drop and several trailing "satellite" drops as they travel from the pen to the paper. Printed characters, for example, are formed by the main drops. The trailing satellites, for the most part, are covered by main drops. However, satellite ink drops which are not covered by main drops, such as satellites along a trailing edge of a printed character, can adversely affect the print quality. These "exposed" satellite ink drops, i.e. those not covered by primary ink drops, appear as roughness along some edges of the printed character. What is needed is a practical way to improve print quality by minimizing edge roughness.
Edge roughness resulting from ink satellites also is a function of the speed at which the pen travels across the surface of the paper. It is exacerbated by increased pen speed. Therefore, decreased print quality must be traded off against increased printing speed. What is needed is a way to allow increased pen speeds, with resulting increased printing speeds, without degrading print quality in terms of edge roughness.